Tire with tread base rubber blend

ABSTRACT

A tire having a tread of a cap/base construction where the base rubber is comprised of at least one selected diene rubber and a trans 1,4-polybutadiene rubber.

FIELD

This invention relates to a pneumatic rubber tire having a rubber tread of a cap/base construction. More specifically, the invention relates to such a tire having a tread base of an elastomeric composition.

BACKGROUND

Pneumatic rubber tires are often prepared with a rubber tread having a cap/base construction. Such constructions are well known.

Typically, the outer cap portion of the tread contains the visible tread grooves and lugs, or raised portions, which is designed to contact the ground. The cap portion usually includes such tread configuration and usually extends to a tread depth of just below the grooves of the tread. The rubber for the cap portion is typically compounded to provide good skid resistance, treadwear and rolling resistance.

The rubber base portion of the tread is positioned beneath the cap portion and, being a part of the tread itself, is located between the outer tread cap and an underlying supporting belt or carcass portion of the tire. Such tire construction is well known. The rubber for the base is typically compounded to enhance rolling resistance and durability for the tire.

Often, a primary purpose for dividing a tread into an outer cap portion and an inner, underlying base portion is to provide a tread base which will reduce the tire's rolling resistance. Otherwise a single composition tread composition might be satisfactory so that the tread is composed of the tread cap throughout.

In one aspect, the cap/base rubber composite may be designed to improve the rolling resistance of the tire without unduly sacrificing its traction (skid resistance) or treadwear. Such often desirable aspect is usually difficult to obtain with a single tread compound because, for example, rolling resistance reduction is typically obtained at the expense of traction and/or treadwear.

In another aspect, it would seem that increasing the thickness (gauge) of the base rubber compound would provide an additional improvement in tire rolling resistance (lower resistance to the rolling of the tire, usually under loaded conditions).

However, it has been observed that increasing the thickness of the base tread rubber can result, during the molding and curing of the tire, in extreme base peaking of the base rubber into the lugs of the cap rubber itself. Thus, elements of the base are caused to extend substantially outward into the cap portion of the tire. This is disadvantageous both because groove cracking in the tread cap may develop and also primarily as the lugs of the cap wear away as the tire is used, the exposed base compound becoming in contact with the road would result in poor traction and treadwear.

A difficulty of such base peaking may become evident as the tire tread wears during use so that the tread cap becomes thinner and the base peaks eventually become exposed and contact the road surface. The resulting exposed tread surface may then not present optimum tread properties to the road surface.

The phenomenon of the base peaking into the lug portion of the cap is largely attributed to the uncured base rubber viscosity often being lower than the viscosity of the cap rubber. Thus, during the mold and curing of the tire under conditions of heat and pressure, the base rubber may have a greater tendency to flow than the cap rubber and, therefore, allow a displacement of the base rubber as the cap rubber is formed into lugs and grooves.

It is, therefore, desirable to increase the viscosity of the uncured base rubber compound while maintaining a satisfactory resilience (rebound value) of the cured base rubber compound.

In one aspect, the viscosity of the uncured base rubber may be readily increased with conventional compounding ingredients, such as, for example, increasing its carbon black content and, optionally, reducing its oil content. However, it is considered that such technique tends to defeat the overall concept of a tread base because it tends to increase hysteresis of the rubber compound which typically results in poorer rolling resistance of the tire.

The use of trans 1,4-polybutadiene has been disclosed for various purposes, including, for example, tire tread rubber compounds and increasing green strength of rubber mixtures (See Japanese Patent Publication Nos. 60-133,036: 62-101,504 and 61-143,453) and U.S. Pat. No. 4,510,291.

Uniquely, trans 1,4-polybutadiene is typically a thermoplastic resin rather than rubber in its uncured state at room temperature by virtue of its high crystallinity. Because it contains many double bonds in its backbone, it can, however, be suitably blended and co-cured with elastomers.

DISCLOSURE AND PRACTICE OF THE INVENTION

In accordance with this invention, a rubber tire is provided having a rubber tread of a cap/base construction where said base is a sulfur cured rubber composition comprised of, based on 100 parts by weight rubber (phr), (A) about 50 to about 95, preferably about 65 to about 90 parts by weight of at least one diene rubber selected from natural and/or synthetic cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber, styrene/butadiene copolymer rubber, styrene/isoprene/butadiene terpolymer rubber and 3,4-polyisoprene rubber; and (B) about 5 to about 50, preferably about 10 to about 35 parts by weight of a trans 1,4-polybutadiene characterized by having at least about a 70 percent trans 1,4-content.

Preferably, such trans 1,4-polybutadiene is characterized by having about 75 to about an 85 percent of its butadiene repeat units of a trans 1,4-isomeric structure, about 12 to about 18 percent of its units of a 1,2-structure and about 3 to about 8 percent of its units of a cis 1,4-structure and, in its uncured state, a first major melting point in the range of about 35° C. to about 45° C. and a second minor melting point in the range of about 55° C. to about 65° C.

In one aspect of the invention, it is preferred that the Mooney (ML1+4) value of the uncured compounded rubber base composition is within about 15 units of such value for said uncured compounded cap rubber composition. Thus, for example, the Mooney (ML1+4) value for said base compounded rubber composition may be in the range of about 30 to about 60 and for the cap compounded rubber compound may be in the range of about 45 to about 75. For the purposes of this description, the "compounded" rubber compositions refer to the respective rubber compositions which have been compounded with appropriate compounding ingredients such as, for example, carbon black, oil, stearic acid, zinc oxide, silica, wax, antidegradants, resin(s), sulfur and accelerator(s).

The addition of the aforesaid trans 1,4-polybutadiene resin to the tread base rubber composition has been observed to enable the use of a 50 percent increase in gauge (thickness) of a tread base, while reducing the gauge of the tread cap by a corresponding amount and, thus, retaining the overall thickness of the cap/base construction, without increasing base peaking in the tread cap/base combination. This would be anticipated to reduce the hysteresis of the cap/base composite and result in improving rolling resistance.

Such phenomenon uniquely suggests the preparation of an improved tire tread by enabling the use of a tread cap with strong high traction and/or treadwear properties with a thicker tread base without appreciably sacrificing rolling resistance.

Thus, a further aspect of this invention is directed to a method of preparing a pneumatic rubber tire with a tread of cap/base construction which comprises shaping and curing an uncured pneumatic rubber tire in a mold by pressing said tire outwardly against a mold surface under conditions of heat and pressure to cause at least the tread rubber of said tire to flow and cure against said mold surface, the improvement which comprises providing, for said tread base, the base rubber of this invention. Preferably, the Mooney (ML1+4) value of the compounded base rubber composition is within 15 units of such value for the compounded cap rubber composition. This, in combination with the use of the trans 1,4-polyisoprene is important to retard the flow, or peaking, of the base rubber into the cap rubber during the tire cure operation.

The relative low melting points of the required trans 1,4-polybutadiene are particularly an advantage because they do not present an appreciable processing difficulty because they are substantially below typical tread rubber processing temperatures, whereas, other potential methods which might be considered for reducing base peaking involve using larger amounts of filler or higher softening point resins would present processing difficulties.

The trans 1,4-polybutadiene utilized by this invention can be prepared by anionic polymerization by batch polymerizing 1,3-butadiene in an organic solvent and in the presence of cobalt octoate and triethyl aluminum as a catalyst system with a para alkyl substituted phenol as a catalyst modifier.

Significantly, the trans 1,4-polybutadiene, in its uncured state, exhibits two distinct melting points: a major melting point and a minor melting point. Preferably, the said first and second melting points are separated by at least 15° C. and usually about 20° C.

In the practice of this invention, the tread cap rubber composition may be comprised of at least one rubber selected, for example, from natural rubber, cis 1,4-polyisoprene rubber, 3,4-polyisoprene rubber, styrene/butadiene copolymer rubbers, styrene/isoprene/butadiene terpolymer rubbers, and cis 1,4-polybutadiene rubber. Preferably it is comprised of a combination of natural rubber or cis 1,4-polyisoprene rubber and cis 1,4-polybutadiene rubbers.

It is readily understood by those having skill in the art that the rubber compositions of the cap and base rubbers would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.

Typical additions of carbon black comprise about 20 to 100 parts by weight of diene rubber (phr), preferably 30 to 60 phr. Typical amounts of tackifier resins, if used, comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids comprise 1 to 20 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Silica, if used, may be used in an amount of about 5 to about 25 phr, often with a silica coupling agent. Representative silicas may be, for example, hydrated amorphous silicas. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to about 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide. The presence and relative amounts of the above additives are considered to be not an aspect of the present invention which is more primarily directed to the utilization of specified blends of rubbers in tire treads, particularly as to the inclusion of the trans polybutadiene in the tread base, as sulfur vulcanizable compositions.

The vulcanization is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, or even, in some circumstances, up to about 8 phr, with a range of from about 1.5 to about 2.25 being preferred.

Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally, a primary accelerator is used in amounts ranging from about 0.5 to about 2.0 phr. In another embodiment, combinations of two or more accelerators which is generally used in the larger amount (0.5 to 1.0 phr), and a secondary accelerator which is generally used in smaller amounts (0.05-0.50 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators have been known to produce a synergistic effect of the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not effected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound. The presence and relative amounts of sulfur vulcanizing agent and accelerator(s) are not considered to be an aspect of this invention which is more primarily directed to the utilization of specified blends of rubbers in tire treads, particularly the inclusion of the trans polybutadiene in the tread base.

The tire can be built, shaped, molded and cured by various methods which will be readily apparent to those having skill in such art.

The prepared tire of this invention is conventionally shaped and cured by methods known to those having skill in such art.

The invention may be better understood by reference to the following example in which the parts and percentages are by weight unless otherwise indicated.

EXAMPLE I

Mixtures of diene rubbers and trans 1,4-polybutadiene having two softening points of about 40° C. and 60° C. were prepared comprised of the following recipe shown in Table 1 as Experiments A and B. Experiment B is considered a control.

                  TABLE 1                                                          ______________________________________                                                            Parts                                                                                    Exp B                                             Material             Exp A   (Control)                                         ______________________________________                                         Natural Rubber       75      75                                                Synthetic Cis 1,4-Polyisoprene                                                                      25      25                                                Trans 1,4-Polybutadiene.sup.1                                                                       15       0                                                Carbon Black         35      35                                                Processing Oil (paraffinic)                                                                          2       2                                                ______________________________________                                    

Conventional preferred, amounts of antidegradant(s) (para-phenylene diamine type), tackifying resin, fatty acid, zinc oxide, peptizer, sulfur and accelerator of the sulfenamide type were used.

The trans 1,4-polybutadiene for this example was characterized by having a trans 1,4-content of about 80 percent, a cis 1,4-content of about 5 percent and a vinyl 1,2-content of about 15 percent. It was further characterized by having an number average molecular weight (Mn) of about 205,000 and a weight average molecular weight (Mw) of about 430,000. It was additionally characterized by having a Tg of about -75° C. and melting points (Tm) of 40° C. (major) and 60° C. (minor). (Both the Tg and Tm were determined by differential scanning calorimeter at 10° C. per minute).

Such trans 1,4-polybutadine can be suitably prepared by batch polymerizing 1,3-butadiene in an aliphatic hydrocarbon solution (e.g. hexane) in the presence of a catalyst of cobalt octoate and triethylaluminum with p-dodecylphenol modifier, although it can also be prepared by continuous polymerization with a suitable gel inhibitor.

EXAMPLE II

The prepared rubber compositions were cured at a temperature of about 150° C. for about 20 minutes and the resulting cured rubber samples evaluated for their physical properties as shown in the following Table 2. The experimental samples A and B correspond to the experimental samples A and B of Example I.

                  TABLE 2                                                          ______________________________________                                         Properties         Ex. A   Ex. B (Control)                                     ______________________________________                                         MPT Viscosity (KPa).sup.1                                                                         15.7    12.3                                                Mooney viscosity.sup.2                                                                            42.5    34.5                                                Rebound.sup.3 (%)  73      73                                                  Tear (Strebler Adhesion).sup.4 (N)                                                                92      87                                                  Low Strain Modulus (MPa).sup.5                                                                    3.0     2.6                                                 ______________________________________                                          .sup.1 A measure of shear viscosity measured by a Monsanto Processability      Tester at constant shear rate conditions.                                      .sup.2 ML(1 plus 4) at 100° C. of the uncured compounded rubber.        .sup.3 The rebound value is determined by pendulum rebound test, a type o      method well known to those having skill in rubber property determination.      It is a cured rubber property.                                                 .sup.4 Adhesion to itself. It is a cured rubber property.                      .sup.5 Tested with a Dynamic Viscoelastic Rheovibron instrument, a testin      instrument by the Toyo Baldwin Company, Ltd.  showing a cured property.  

Thus, the viscosities of the uncured rubber are significantly higher for Exp. A, while their cured properties, rebound tear and modulus are similar, 1,4-polybutadiene is considered an excellent candidate for tread base application.

EXAMPLE III

Mixtures of diene rubbers and trans 1,4-polybutadiene having two softening points of about 40° C. and about 60° C. were prepared comprised of the recipe of Example I, with modifications, shown in Table 3 as Experiments B and C. Experiment B is considered a control and is similar or equivalent to Control B of Example I.

                  TABLE 3                                                          ______________________________________                                                         Parts                                                          Material          Exp B (Control)                                                                              Exp C                                          ______________________________________                                         Natural Rubber    75            75                                             Synthetic Cis 1,4-Polyisoprene                                                                   25             0                                             Trans 1,4-Polybutadiene.sup.1                                                                     0            25                                             Carbon Black      35            35                                             Processing Oil (paraffinic)                                                                       2             2                                             ______________________________________                                         Conventional, preferred, amounts of antidegradant(s)                           (para-phenylene diamine type), tackifying resin, fatty acid,                   wax, zinc oxide, peptizer, sulfur and accelerator                              of the sulfenamide type were used.                                              .sup.1 A trans 1,4polybutadiene for this invention characterized by high       trans 1,4polybutadiene content (80 percent trans 1,4).                   

Conventional, preferred, amounts of antidegradant(s) (para-phenylene diamine type), tackifying resin, fatty acid, wax, zinc oxide, peptizer, sulfur and accelerator of the sulfenamide type were used.

EXAMPLE IV

The prepared rubber compositions of Example III were cured at a temperature of about 150° C. for about 20 minutes and the resulting cured rubber samples evaluated for their physical properties as shown in the following Table 2. The experimental samples B and C correspond to the experimental samples B and C of Example III.

                  TABLE 4                                                          ______________________________________                                                         Parts                                                          Material          Exp B (Control)                                                                              Exp C                                          ______________________________________                                         Tensile Strength (MPa)                                                                           21.7          19.8                                           Modulus (300%) (MPa)                                                                             8.9           9.1                                            Elongation (percent)                                                                             560           520                                            Rebound.sup.1 (%) 72            72                                             Tear (Strebler Adhesion).sup.2 (N)                                                               40            24                                             Low Strain Modulus (MPa).sup.3                                                                   3.8           4.1                                            ML small (Mooney Viscosity).sup.4                                                                21.8          30.2                                           Spider Mold Flow.sup.5 (mm)                                                                      100           50                                             Green Strength.sup.6 (MPa)                                                     @ 40% elongation  4.2           10.2                                           @ 600% elongation 7.2           54                                             ______________________________________                                          .sup.1 The rebound value is determined by pendulum rebound test, a type o      method well known to those having skill in rubber property determination.      .sup.2 A measure of adhesion to itself.                                        .sup.3 Measured at 0.5% elongation. Tested with Rheovibron instrument.         .sup.4 Mooney small, M5(1 + 4), at 100° C. of the uncured               compounded rubber.                                                             .sup.5 A measure of mold flow under heat and pressure  see Du Pont Mold        Flow example in the Vanderbilt Rubber Handbook (1978, page 507).               .sup.6 Measure of stress of the uncured rubber compound.                 

Observation of the physical properties of the rubber composite demonstrate that uncured compound viscosity increased while maintaining a satisfactory resilience (rebound value) of the cured compound.

EXAMPLE V

Pneumatic rubber tires were prepared of size P195/75R14, steel belted radial ply tires, which had rubber treads of a cap/base construction. Two tires had their base rubber composed of the rubber composition shown as Experiment C of Example III, and two tires had their base rubber composed of the control Experiment B of Experiment III.

The tires are correspondingly identified herein as Tires B-1 and B-2 and Tires C-1 and C-2, respectively. Tires B-1 and C-1 had tread bases of green thicknesses of 0.05 inches and tires B-2 and C-2 had tread bases of green (uncured) thicknesses of 0.075 inches.

Cross-sections of the cured tires were obtained and their tread cap/base interfaces visually inspected.

A significant reduction of base peaking into the tread cap was observed for tires with treads having a base composition of said Experiment C as compared to the tire treads with a base composition of Control Experiment B.

These tires demonstrate that the inclusion of the trans 1,4-polybutadiene in the tread has a very definite effect in reducing base peaking into the tread cap.

Indeed, the visible effect is more dramatic with Tire B-2 (Control) and Tire C-2 (using the trans 1,4-polybutadiene in the tread base). These tires had the thicker tread base and, thus, would normally be expected to exhibit a greater degree of base peaking into the tread cap. For the Control Tire B-2, there was the considerable and expected base peaking. For the Experimental Tire C-2, there was little or no base peaking.

Therefore, it is observed that a particular advantage of this invention, where a tire tread composed of an outer cap portion and an inner, underlying, base portion is the utilization of trans 1,4-polybutadiene rubber in the tread base rubber composition to retard base peaking into the tread cap during the tire molding and curing operation.

This aspect can be of particular advantage where a thicker tread base is desired in a tread cap/base construction where the base is more resilient and, thus, contributes to desirably reducing the rolling resistance of the tire itself.

Thus, in the practice of this invention, the trans 1,4-polybutadiene polymer is considered as acting as a reinforcing plastic filler before vulcanization and restricts flow of the tread base rubber into the cap rubber during the molding of the tire, however, during curing the trans 1,4-polybutadiene plastic melts and blends into the polymer matrix, transforms into a rubbery polymer upon curing resulting in a highly resilient cured polymer blend.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention. 

What is claimed is:
 1. A rubber tire having a rubber tread of a cap/base construction where said base rubber composition is comprised of, based on 100 parts by weight rubber, (A) about 50 to about 95 parts by weight of at least one diene rubber selected from natural and/or synthetic cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber, styrene/butadiene copolymer rubber, styrene/isoprene/butadiene terpolymer rubber and 3,4-polyisoprene rubber: and (B) about 5 to about 50 parts by weight of a trans 1,4-polybutadiene rubber characterized by having at least about a 70 percent trans 1,4-content.
 2. The tire of claim 1 where said base rubber composition is comprised of, based on 100 parts by weight rubber, (A) about 65 to about 90 parts by weight of at least one of said diene rubbers, and (B) about 10 to about 35 parts by weight of said trans 1,4-polybutadiene rubber.
 3. The tire of claim 2 where the Mooney (ML1+4) values of the uncured compounded base rubber composition is less than and such value for the uncured compounded cap rubber composition.
 4. A rubber tire having a rubber tread of a cap/base construction where said base rubber composition is comprised of, based on 100 parts by weight rubber, (A) about 50 to about 95 parts by weight of at least one diene rubber selected from natural and/or synthetic cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber, styrene/butadiene copolymer rubber, styrene/isoprene/butadiene terpolymer rubber and 3,4-polyisoprene rubber; and (B) about 5 to about 50 parts by weight of a trans 1,4-polybutadiene rubber characterized by having about a 75 to about a 85 percent trans 1,4-content, about a 12 to about an 18 percent 1,2-content and about a 3 to about an 8 percent cis 1,4-content and, in its uncured state, a first major melting point in the range of about 35° C. to about 45° C. and a second minor melting point in the range of about 55° C. to about 65° C.
 5. The tire of claim 4 where said base rubber composition is comprised of, based on 100 parts by weight rubber, (A) about 65 to about 90 parts by weight of at least one of said diene rubbers, and (B) about 10 to about 35 parts by weight of said trans 1,4-polybutadiene rubber.
 6. The tire of claim 4 where the Mooney (ML1+4) value of the uncured compounded base rubber composition is within about 15 units of such value for said uncured compounded cap rubber composition.
 7. The tire of claim 6 wherein the Mooney (ML1+4) value for said uncured base rubber composition is in the range of about 30 to about 60 and for said uncured cap rubber compound is in the range of about 45 to about
 75. 8. A method of preparing a pneumatic rubber tire with a rubber tread of a cap/base construction which comprises shaping and curing an uncured pneumatic rubber tire in a mold by pressing said tire outwardly against a mold surface under conditions of heat and pressure to cause at least the tread rubber of said tire to flow and cure against said mold surface, the improvement which said rubber tread has a base rubber composition of, based on 100 parts by weight rubber, (A) about 50 to about 95 parts by weight of at least one diene rubber selected from natural and/or synthetic cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber, styrene/butadiene copolymer rubber, styrene/isoprene/butadiene terpolymer rubber and 3,4-polyisoprene rubber; and (B) about 5 to about 50 parts by weight of a trans 1,4-polybutadieen rubber characterized by having at least about a 70 percent trans 1,4-content.
 9. The method of claim 8 in which the base rubber composition of the tread of the said pneumatic tire is comprised of, based on 100 parts by weight rubber, (A) about 65 to about 90 parts by weight of at least one of said diene rubbers, and (B) about 10 to about 35 parts by weight of said trans 1,4-polybutadiene rubber.
 10. The method of claim 8 in which the base rubber composition of the tread of the said pneumatic tire is comprised of, based on 100 parts by weight rubber, (A) about 50 to about 95 parts by weight of at least one diene rubber selected from natural and/or synthetic cis 1,4-polyisoprene rubber, cis 1,4-polybutadiene rubber, styrene/butadiene copolymer rubber, styrene/isoprene/butadiene terpolymer rubber and 3,4-polyisoprene rubber; and (B) about 5 to about 50 parts by weight of a trans 1,4-polybutadiee rubber characterized by having about a 75 to about a 85 percent trans 1,4-content and about a 3 to about an 8 percent cis 1,4-content and, in its uncured state, a first major melting point in the range of about 35° C. to about 45° C. and a second minor melting point in the range of about 55° C. to about 65° C.
 11. The method of claim 8 where, for the rubber tread of the said pneumatic tire, the Mooney (ML1+4) value of the uncured compounded base rubber composition is within 15 units of such value for uncured compounded cap rubber composition.
 12. The method of claim 8 where, for the rubber tread of the said pneumatic tire, the Mooney (ML1+4) value for said uncured base rubber composition is in the range of about 30 to about 60 and for the uncured cap rubber compound is in the range of about 45 to about
 75. 